A tunable filter uses electrically controlled liquid crystal (LC) elements to transmit specific wavelengths of light through the filter by exploiting the variable retardation associated with certain LC modes as a means of shifting a spectral feature. One type of LC tunable filter is a Lyot (or Lyot-hybrid) polarization interference filter. With a Lyot filter, a bandpass profile is synthesized through multistage filtering using geometric relationships between retarder stack films. The polarization analyzer of one stage forms the input polarization for a subsequent stage, such that (N+1) polarizing films are used for an N-stage filter. In other word, 2 polarizing films are used for 1 filter stage, 3 polarizing films for 2 filter stages, and so forth. The overhead associated with calibrating a fully tunable Lyot bandpass filter to provide acceptable spectral characteristics can be significant. Additionally, each polarizing film has approximately a 10% transmission loss. Consequently, tunable Lyot filters with high finesse and acceptable dynamic ranges are not only bulky and expensive, but also have poor peak transmission.
A Solc or Solc-like filter, on the other hand, can be synthesized using only two polarizers bonded to a single retarder stack. Also, the Solc filter can, in principle, be customized to reduce side-lobe levels. Presently known bandpass-tuning Solc filter approaches, however, require that each multi-order retarder stack be fully tunable. In practice, there is no significant improvement in throughput because the insertion loss of a polarizer is traded for the additional LC cell loss. Furthermore, the construction of a Solc filter is more challenging than that of a Lyot filter, in that precise alignment of many interleaved active and passive elements must be done before system calibration can commence. Consequently, if at any point an error is made in building such an assembly, a large amount of high value material may have to be scrapped.